iio: imu: inv_mpu6050: reduce sleep time when turning regulators on

Turning vdd regulator on requires a consequent sleep for the
chip to power on correctly.
Turning vddio regulator is much faster.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>